Data for: Long-term drought reduces wildfire severity and promotes invasive species

Description

Notes

Methods

The Loma Ridge manipulation consists of replicate experimental blocks established in CSS and grassland communities. Each experimental block includes 6 plots, with each plot randomly assigned to a unique water (droughted, ambient, and added) and nitrogen (ambient or added) manipulation treatment. The drought involves an approximately 40% reduction in precipitation, achieved by covering plots with plastic during large storms. Water is funneled into storage tanks and is later pumped on water addition plots that receive approximately 30% more water through irrigation lines.

To measure plant community composition, each plot was divided into three 4x4m quadrats, within which all species were identified and % cover was visually estimated. For all shrubs, separate cover values were recorded for crown-sprouting individuals, dead shrubs, and seedlings. Overlapping plants meant that plant cover could total >100%. Ground cover values (bare ground, thatch, litter, cryptobiotic crust, rock, or moss) were separately estimated so that they totaled 100%.

We collected herbaceous biomass and litter within four 14 cm by 50 cm sampling frames per plot each April. Within each of these frames, the cover of any rooted plants, along with ground cover, was recorded by functional group (forbs, grasses, shrubs, litter, or bare ground) to total 100%. It was extremely rare for shrubs to be rooted within this small frame, so shrub cover was typically close to 0%. After estimating cover, all litter within each frame was collected and all herbaceous material was cut and harvested. Material was dried in an oven at 60°C degrees for 4 days and weighed. The herbaceous mass measurements include separate and combined totals of all living biomass and dead (litter) mass within each sampling frame. Shrub biomass was estimated pre-fire by measuring all shrubs greater than 20 cm in height that were rooted within the 4x4 m subplot. Total height (H, height at the tallest point), width 1 (W1, width at the widest point) and width 2 (W2, width perpendicular to width1) were used to estimate total shrub volume (V=H*pi(W1*W2/4)). Biomass was estimated from volume using regression equations developed for each species or generalized shrub regression equations (Vourlitis and Pasquini 2009).

After fire in Spring 2021, impacts of fire severity on CSS were measured in each of the 24 subplots representing added, ambient, and reduced precipitation with ambient nitrogen. We measured three individuals of each of the three most abundant species (Malosma laurina, Artemisia californica, and Salvia mellifera) in each plot. For each individual shrub, we counted the number of branches remaining, the percentage of branches with leaves, the diameter of the smallest terminal branch (branch width), the height of the burned shrub, the number of burned shrubs that were crown sprouting, and the height of the basal crown sprouting new growth. Higher fire severity results in lower branch count and leaf percent cover, and greater branch thickness (Perez and Moreno 1998).